1. Field of the Invention
This invention relates to a novel process for vaporizing liquid reagents for use in CVD reactions. The invention further relates to an apparatus and method for creating a vapor containing controlled amounts of suitably reactive chemicals.
2. Description of the Related Art
Chemical vapor deposition (CVD) is a widely-used process for forming solid materials, such as coatings or powders, from reactants in the vapor phase. Reviews of CVD processes have been published recently in "CVD of Nonmetals," W. S. Rees, Jr., Editor, VCH Publishers, Weinheim, Germany, 1996; "CVD of Compound Semiconductors," A. C. Jones and P. O'Brien, VCH, 1996; and "The Chemistry of Metal CVD," T. Kodas and M. Hampden-Smith, Editors, VCH, 1994. In CVD processes, a reactant vapor may be created by heating a liquid to a sufficiently high temperature and bubbling a flow of a carrier gas through the liquid, to transport the vapor into the CVD chamber. In a low-pressure CVD system, the carrier gas may be omitted, and the vapor may flow directly from the bubbler into the low-pressure CVD chamber.
Thermal decomposition of materials in heated bubblers can reduce the reproducibility and purity of vapors delivered from bubblers. Thermal decomposition may be minimized by rapid or "flash" vaporization. This can be accomplished by pumping the liquid at a steady, controlled rate onto a hot surface from which the liquid vaporizes quickly. In such a "direct liquid injection" (DLI) system, each part of the liquid is heated for only a short time, and its vapor can be formed without significant decomposition even from thermally sensitive liquids. Another advantage of a DLI system is that multicomponent mixtures can be vaporized in a fixed and reproducible ratio, even if the components differ in volatility. Commercial DLI systems are produced by NIKS Instruments (Andover, Mass.), Advanced Technology Materials (Danbury, Conn.), Novellus Systems, (San Jose, Calif.), COVA Technologies (Tiburton, Calif.), [and] Artisan Industries (Waltham, Massachusetts) and Bronkhorst Hi-Tec B. V. (Ruurlo, Netherlands).
These DLI systems have some disadvantages, in that the heated surfaces can catalyze the decomposition of the liquid precursor as it is evaporating. Decomposition products can build up on the heated surfaces and eventually require cleaning or replacement of parts of the vaporizer. Also, some of them produce a significant pressure drop in the carrier gas flow.
Solids can also be used as sources of vapor in CVD processes. However, when solids are used in a bubbler, the rate of vapor production by sublimation of a solid is not easily reproducible, because the amount of vapor produced often depends on the particle size and shape, which change as the sublimation process continues. Thus the vapor concentration can change in an uncontrolled way, thereby changing the growth rate and/or the composition of materials made by the CVD process. Also, different batches of solid may have different sizes and shapes of particles, so that the results of a CVD process may change when a new batch of solid precursor is placed in the system.
Solid sources can be used in DLI vapor sources if a suitable liquid solvent can be found to dissolve the solid. However, solvents can introduce other difficulties, such as increased flammability, toxicity or corrosiveness, and an increased volume of gaseous byproducts, which must be removed from the exhaust gases to avoid pollution. When solutions are introduced into a DLI system, the solvent can evaporate, leaving a solid residue which builds up on the heated surface, and may decompose there, rather than vaporize.
Ultrasonic nebulization has been used to overcome some of these difficulties with vaporization of solutions. For example, the Pyrosol process (Sepr Co., France) applies) ultrasonic power to a large container of solution, causing a spray of droplets to leave from its surface. However, the rate at which the solution is nebulized in the Pyrosol process is not easily controlled, because it is dependent on the composition and temperature of the solution, and the depth to which its container is filled. It takes many minutes to stabilize the rate of nebulization, during which time the temperature of the solution rises to a steady-state value because of the high ultrasonic power put into the solution, and the concentration of the solution rises due to evaporation of part of the solvent.